Nothing in the TN rules dictates who pays. The application fee at the port of entry or the I-129 fee at USCIS is set by the USCIS fee schedule, and professional fees are a matter of agreement. The offer should state who pays and whether any repayment clause applies if the applicant leaves.
Put the cost allocation into the offer letter
Hypothetical example: the distributor's first offer to the West Kelowna CPA is silent on immigration costs, and the recruiter says the company usually reimburses. A reimbursement promise made verbally is not a term of employment; it should appear in the signed offer with the amount, the timing and any condition. Employers occasionally add a clawback requiring repayment of relocation and immigration costs if the employee resigns within a period.
Whether such a clause is enforceable is a matter of state employment law, not immigration law, but its existence affects the applicant's exposure and should be read before acceptance. Government charges are published in the USCIS fee schedule, which is where both parties should look rather than relying on a figure someone remembers from a previous hire. Professional fees for preparing the file are agreed separately and are not a government requirement.
A bonus paid in the first cheque is compensation, not a reimbursement, and does not settle the cost question unless the offer says it does.